A steel sheet used for an automobile and an industrial machine by press-forming is required to have both a high strength and a high workability (good balance between strength and elongation) from the viewpoint of the improvement of collision safety and the improvement of fuel efficiency and the weight reduction of a vehicle body accompanied by environmental issues. As a high-strength steel sheet excellent in workability, a TRIP steel sheet is used. The TRIP steel sheet is a steel sheet in which an austenitic structure is retained, the retained austenite (γR) is induced-transformed into martensite by stress and strain, and thereby a large elongation is obtained.
In the meantime, an automobile structural member such as a member to absorb collision energy is required to have an excellent shape freezing property in bending or hat-shaped bending work in addition to the above properties. The shape freezing property means the property of freezing (preventing) the change of the shape caused by springback after a steel sheet is worked.
A problem however is that in general, as the strength of a steel sheet increases, the springback value increases after working and the shape freezing property deteriorates. In a TRIP steel sheet in particular, it is said that, since portions where retained austenite transforms into martensite and portions where retained austenite does not transform into martensite appear unevenly in the interior of the steel sheet after forming, a large residual stress is generated and a springback value increases.
Consequently, studies have been worked on in order to provide a TRIP steel sheet having a higher shape freezing property while maintaining a good workability.
For example, JP-A No. 61326/1999 discloses that a work hardening coefficient (an n value in 5% to 10% strain) of a steel sheet is useful as an index of the collision safety of an automobile member and, by controlling the average crystal grain size of retained austenite to 5 μm or less, it is possible to obtain a high strength and a high elongation (TS×EL≧20,000) and provide a TRIP steel sheet having a high n value.
JP-A No. 154283/2007 discloses a high-strength steel sheet in which the springback value is low and the residual stress after forming is lower than ever before while a high formability is maintained by mainly comprising a ferrite phase and an austenite phase of 3% or more and controlling the ratio of the portion having an aspect ratio of 2.5 or less in crystal grains at the portion other than the ferrite phase.
The present applicants also disclose technologies in JP-A Nos. 350064/1999 and 218025/2004, for example. In JP-A No. 350064/1999, a TRIP steel sheet in which the steel sheet comprises the three phases of ferrite, martensite, and 1% to 5% retained austenite and the hardness of the martensite is controlled is disclosed. Then in JP-A No. 218025/2004, a TRIP steel sheet having a combined structure comprising tempered martensite and ferrite as the mother phase in which the quantity of retained austenite that transforms into martensite by applying 2% strain in the retained austenite (retained austenite that has a low C content and unstable in the retained austenite) is precisely controlled is disclosed.